PDE-5 inhibitors are a class of medications commonly used for the treatment of erectile dysfunction (ED). They work by blocking the action of an enzyme called phosphodiesterase type 5 (PDE-5), which is responsible for breaking down cyclic guanosine monophosphate (cGMP) in the smooth muscle cells of the penis. By inhibiting PDE-5, these medications increase the levels of cGMP, which leads to relaxation of smooth muscle cells and increased blood flow to the penis, resulting in improved erectile function.